Rapid Spanning-Tree Configuration

In a previous lesson I explained the differences between classic and rapid spanning-tree and how rapid spanning-tree works. If you haven’t seen it before, I would recommend to look at it first before diving in the configuration.

Having said that, let’s look at the configuration. This is the topology that I will use:

This is the topology I’m going to use. SW1 will be the root bridge in my example. First we have to enable rapid spanning-tree:

SW1(config)#spanning-tree mode rapid-pvst

SW2(config)#spanning-tree mode rapid-pvst

SW3(config)#spanning-tree mode rapid-pvst

That’s it…just one command will enable rapid spanning tree on our switches. The implementation of rapid spanning tree is rapid-pvst. We are calculating a rapid spanning tree for each VLAN.

Apparently SW2 thought it was the root bridge because it says it received a superior BPDU on its fa0/14 interface. It changes its fa0/14 interface to root port.

SW2# RSTP(1): syncing port Fa0/16

The fa0/16 interface on SW2 will go into sync mode. This is the interface that connects to SW3.

SW2# RSTP(1): synced Fa0/14
RSTP(1): transmitting an agreement on Fa0/14 as a response to a proposal

SW2 will respond to SW1 its proposal by sending an agreement.

SW1# RSTP(1): received an agreement on Fa0/14
%LINK-3-UPDOWN: Interface FastEthernet0/14, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/14, changed state to up

SW1 receives the agreement from SW2 and interface fa0/14 will go into forwarding.

SW2# RSTP(1): transmitting a proposal on Fa0/16

SW2 will send a proposal to SW3.

SW3# RSTP(1): transmitting an agreement on Fa0/16 as a response to a proposal

SW3 will respond to the proposal of SW2 and send an agreement.

SW2# RSTP(1): received an agreement on Fa0/16
%LINK-3-UPDOWN: Interface FastEthernet0/14, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/14, changed state to up

SW2 receives the agreement from SW3 and the interface will go into forwarding. That’s all there to is it…a quick number of handshakes and the interfaces will move to forwarding without the use of any timers. Let’s continue!

SW1(config)#interface fa0/17
SW1(config-if)#no shutdown

I’m going to enable this interface so that connectivity is fully restored. Let’s look at an overview:

We can verify that SW1 is the root bridge. This show command also reveals that we are running rapid spanning tree. Note that the link type is p2p. This is because my FastEthernet interfaces are in full duplex by default. Let’s run the same command on the other two switches:

SW3 realized something is wrong with the root port almost immediately and will change the fa0/16 interface from alternate port to root port. This is the equivalent of UplinkFast for classic spanning tree but it’s enabled by default for rapid spanning tree.

Forum Replies

Rene,
Hi. In the last part of the lesson you convert switch B to PVST mode - what is the impact to the network - does computer A lose connectivity to the rest of the network outside of switch B for 30 seconds? What about if there was a host connected off of switch A and another host connected off switch B - they would still continue to communicate throughout the change of switch B to PVST mode?

Sorry, Shanmugasiva,
I am not understanding what you are asking. Could you ask this another way?

If you are asking what this message means, I can try to help.setting bridge id (which=3) prio 4097 prio cfg 4096 sysid 1 (on) id 1001.0011.bb0b.3600

“cfg 4096”
This means you set (either via spanning-tree vlan 1 4096, or spanning-tree vlan 1 root primary). This is what it means when it says “prio cfg 4096”–the vlan was configured to be priority 4096.

“prio 4097”
However, the actual priority is the configured priority + vlan number. So, in this case, the actual p

Hello Rene,
I have configured a lab connecting three switches on gns3 (SW1,SW2 and SW3). SW1 is the Root switch. They are connected like this:
SW1(e1/3-DP)>>>SW2(e1/3-RP)
SW1(e3/0-DP)>>>SW3(e3/2-RP)
SW2(e3/0-DP)>>>SW3(e3/3-ALT/BLK)
The issue is that as soon as i admin down the port on SW3(e3/2-RP), the other port e3/3 becomes the new RP. This is expected. But when i bring back the port up on SW3(e3/2), the port doesn’t becomes automatically RP again, because its far end device port SW1 (e3/0) is going into err-disabled state, essentially the moment that i admin

Ok i think i have found the issue here. I have configured udld mode aggressive on all the ports. So that is why when i admin down the port on SW3(e3/2-RP), the Root switch port SW1 (e3/0) goes into err-disable state! Since Root Switch port e3/0 will not receive any traffic, it goes into err-disable state. Finally, i just needed to bounce it, to work as before.

Also, when i tried to admin down the port on the Root Switch (SW1-e3/0) i noticed that the far end port doesn’t go to the error-disable state, even though the far end port (SW3-e3/2) is configured with ud